Add --buildtype=release and -Db_ndebug=true to mesa builds

The pmOS mesa packages need to have -Db_ndebug=true passed to the meson build so that debug assertions and extra validations are disabled to improve overall system performance. See https://gitlab.freedesktop.org/mesa/mesa/blob/master/docs/meson.html#L321 for more details. Fedora also recently made this change: https://bugzilla.redhat.com/show_bug.cgi?id=1692426.

This change, along with using the master branch of mesa, gets the Freedreno driver working properly with X11 on the Nexus 5 and presumably other A3XX-based systems.

--buildtype=release should also be passed to meson for extra optimizations.

See the thread https://lists.freedesktop.org/archives/mesa-dev/2019-June/219960.html for more details.